Passive heliostat

sun tracking equipment represents a significant capital investment for concentrating solar power stations. This is an idea for a passive heliostat for sun tracking in 2d, (solar trough systems)

It's fairly simple, use expansion and contraction of materials to move a solar trough back and forth to track the sun. the system comprises three parts, firstly a disc attached to the end of each trough, a partition attached along the optical axis of the trough to said disc, another disc beside this one but attacked to something non moving finally, two black flexible bags containing a gas that expands quite a bit when heated. the ends of which would be attached to the base of the partition and the outer edges of the fixed disc. idealy the partition shields one of the fluid bags while allowinbg sun to hit the other. this pushed the trough towards the sun so as to, hopefully track it. there would probably be problems with this setup, any comments are appreciated.